Selenium rectifier and method of making



Aug. 26, 1947. c.'s. SMITH, JR 2,426,377

' SELENIUM RECTIFIER AND METHOD OF MAKING Filed Dec. 7, 1943 2Sheets-Sheet 2 Patented Aug. 26, 1947 2,426,377 SELENIUM RECTIFIER ANDMETHOD MAKING Cleveland Scudder Smith, Jr., Indianapolis, Ind., assignorto Samuel Ruben, New Rochelle,

Application December 7, 1943, Serial No. 513,265

11 Claims.

This invention relates to metalrectiflers and photocells, and to methodsand apparatus for depositing a relatively thin layer or layers of ametal such as selenium upon a base composed of a different metal oralloy. It has long been recognized that When the surfaces of certainmetals or metallic compounds are placed in contact with each other suchcombination possesses photoelectric characteristics and is also capableof rectifying an alternating electric current. While numerouscombinations of metals and metallic compounds have been proposed for useas rectifying and photoelectric elements, one of the most satisfactorycombinations is a layer or lm oi selenium deposited upon an aluminum,bronze, copper or ferrous metal base and having a counter-electrodedeposited upon or in Contact with its exposed surface. However, due toits inherent chemical characteristics selenium is difcult to control andof its several known forms which exist at room temperature only one formappears to possess photoelectric and rectifying properties. To besuitable for use in a photoelectric or rectifying element, selenium mustbe present substantially or entirely in .the form cf a thin graymetallic'lm upon a suitable base metal. If the conditions for depositingthe selenium are not carefully controlled, a useless deposit of theamorphous or crystalline form of selenium may result.

Various methods for the deposition of a layer of selenium on a basemetal for the manufacture of rectiers and photocells have been proposed.In general, such processes involve the application of selenium, eitherin paste or powder form, to a previously prepared base plate while thelatter is maintained at an elevated temperature. Another suitably heatedmetal plate is then brought in contact with the paste, and the assemblyis subjected to a hot pressing operation to form a structure having aninner layer of metallic selenium. This method possesses several inherentdisadvantages which render it impractical for commercial use. Forinstance, a layer cf selenium cannot easily be formed which hascontrolled variation in chemical or electrical characteristicsthroughout the body of the selenium, furthermore, several successivelayers cannot readily be built up to form a homogeneous larger layer. Inaddition, such known process is not only costly in time required forcarrying out the numerous steps, but it is impractical for thecommercialmanufacture'of large plate rectiflers oecause of unavoidable variationsin plate thickness, the random orientation of the resulting crystallinedeposit, and the erratic and poor rectifying characteristics of suchsurface. t

Another process has been proposed in which the plates to be coated withselenium are passed the discharge orices of one or more selenium boilersor are mounted at xed distances from stationary selenium boilers, theplates and boiler or boilers being disposed in an evacuated chambermaintained at a pressure of the order of 1 micron of mercury or less.Such, process is necessarily a batch process and is not adapted to therapid and continuous production of photocell and rectifier elements.Also, because of the long pumping time required to obtain the desiredvacuum, and because of practical limitations on the size of the vacuumsystem, such process is highly uneconomical for the manufacture of largesize elements or for the deposition of more than one layer of seleniumboth in first cost, operating cost and maintenance cost.

The present invention overcomes the numerous disadvantages inherent inprior processes and provides a method and apparatus for the economicaland commercial production of selenium photocell and rectier elements.

Accordingly, among the objects of this invention are: to provide animproved method in which selenium is deposited on a base plate underordinary atmospheric conditions in the form of as many different layersof metallic selenium as may be desired; to provide a method in which theselenium deposition is uniform and the thickness of such deposit may beaccurately controlled to any desired value; to provide a method which isadapted to the automatic, continuous and elllcient production ofselenium photocell and rectifier elements; to provide apparatus ofrelatively simple design which is adapted to carry out the method of theinvention.

The above and other objects, advantages and novel features of theinvention will become apparent from the following description andaccompanying drawings, in which:

Fig, 1 shows an elevational view, in cross-section, of one form of anapparatus embodying the principles of this invention;

Fig. 2 is an enlarged sectional view taken from the left along the line2-2 of Fig. 1;

Fig. 3 is an elevational View, partly in crosssection, of a modificationof the apparatus of the invention,

Fig. 4 is an enlarged sectional view of one of the several seleniumvaporizing boilers or units illustrated generally in Fig. 3;

Figs. 5 and 6 are elevational views of a further modification of theselenium vaporizing boilers or units embodying the principles of thisinvention;

Fig. '7 is an elevational view, partly in section of an apparatusadapted to coat a film or thin layer of selenium simultaneously on bothsides of a metal plate;

Fig, 8 illustrates a method 0f applying a metal counter-electrode onto ametallic plate; and

Figs. 9 and 10 illustrate partially completed rectifier blanks.

In general, the improved method of this invention is carried out byvaporizing selenium in a suitable boiler above and substantially atatmospheric pressure and impinging the resulting vapor against thesurface of a suitable metal plate maintained either at or above roomtemperature. The boiler is preferably provided, in one embodiment of theinvention, with a slotted discharge orifice which has a length greaterthan or substantially equal to the width of the plate to be coated. Themetal plate to be coated is maintained at apreselected distance from theorifice of the boiler and when such plate is moved back and forththrough the stream of selenium vapor issuing from such orifice, at apreselected rate and in a direction substantially transversely of thelongitudinal direction of the slot orifice, an extremely smooth anduniform illm or layer of gray metallic selenium of desirable rectifieror photocell quality is deposited on the metal plate. By making severalpasses back and forth through the stream of vapors issuing from theoriiice, any desired thickness of selenium may be built up. If desired,several boilers may be disposed in tandem relation, so that the metallicplate may pass successively through several streams of selenium andthereby quickly build-up a plurality of layers of selenium. In such anarrangement the boilers may be operated to discharge the same ordifferent concentrations of vapors, and different impurities may beintroduced into the several boilers to build up selenium of desirablecomposition for low forward resistance and high reverse resistance,

Referring to the drawings, Figs. 1 and 2 illustrate an embodiment of theinvention wherein a selenium 'boiler B comprising a cylindrical casingI2 is provided with an upper threaded portion I4 adapted to frlctionallyengage a threaded portion I6 of a cap I8. A slotted orifice 20 isvprovided in cap I8, and has substantially uniform cross-sectionthroughout its length. The oriiice 20 has a length substantially equalto the inside diameter of the casing I2. A circular ilange 22 is weldedas at 23, or otherwise secured, to the cap I8 to support the boiler andto position the upper surface of the cap I8 substantially ilush with theupper surface of a suitable supporting base board 24', which may becomposed of asbestos or other non-inflammable material. Guides 26 aresecured to the base board 24 along lines equidistant on either side ofthe boiler B. These guides 26 are adapted to support a slide 28 carryingthe metal plates 30 to be coated with selenium. The metal plates 30 maybe bolted at 32 or otherwise secured to the carriage 28, and cooperatewith suitable grooves 34 in the guides 26. A knob or handle |28 isprovided to slide the plates back and forth on the boiler. The distancebetween the top of the cap I8 and the surface of the metal plate to becoated with selenium, is maintained at a Value such that when seleniumvapor streams at the desired rate through the slotted orice the lowersurface of the metallic plate is uniformly coated. A pair of gas burners36 or other heating agents may be used to vaporize the selenium, and inorder to prevent condensation of selenium vapor in the upper part of thebolier B, the heat is preferably applied to the upper regions of theboiler.

The method of carrying out the invention to obtain a coating of seleniumupon a metal plate is as follows: Boiler casing I2 is unscrewed from capI8, and about 10 grams or more of selenium is deposited in the boiler;the casing is then screwed back into position in the cap I8, andsuitably heated as by one or more gas burners to a temperaturesuflicient to cause a stream of selenium vapor to issue through theorifice 20. Heat is applied to the upper portion of the boiler to avoidcondensation of selenium therein and is conducted downward by the boilerWalls to vaporize the selenium charge. When selenium vapor is issuing atthe desired rate from the orifice, the slide or carriage 28, carryingthe metal plates to be coated, is inserted in the guides and a number ofpasses are made back and forth through the stream of selenium vaporthereby forcing a thin adherent coating of selenium upon the surface ofthe plates.

It has been found that during each deposition of selenium thin iilms areformed on the previous surface and on the new selenium surface, whichiilms are composed of a mixture of selenium oxides, water and condensedparticles of' selenium in forms unsuitable for rectifiers. These lms maycause the selenium to peel or crack, and they may produce high forwardresistance in the completed rectifier.

Usually the interspersed iilms are so thin as to cause no adverseeffects on the completed rectifier. The iilms can be completelyeliminated by conducting the deposition of selenium in an atmosphere ofinert gas such as argon, a relatively inert gas such as nitrogen, orcarbon dioxide, or a reducing gas such as household cooking gas. Thesurrounding gas may be at any desired temperature, and should be abovethe boiling temperature of selenium if it is desired to avoid anycondensation of selenium before the selenium vapor reaches the receivingplate.

The ratio of length to width of the orifice is somewhat critical to thedeposition of the desired layer of selenium. For example, an orificewidth of about 0.012 inch has been found to yield good results. Asmaller width gives a greater proportion of selenium oxide to selenium.Slight variations in the width of the orifice slit are to be avoidedbecause such variations cause striations to appear in the depositedselenium. A greater orifice width makes it diiiicult to build upsufficient pressure in the boiler to permit uniform vapor flow along theentire length of the orice slit, and also causes a greater quantity ofselenium vapor to emerge than is necessary or desirable. Hence, care inconstruction and choice of materials may dictate the use of smallerorifice widths.

The distance of the plate from the discharge orifice must be carefullyselected to give the most desirable results. In general, the closer theplate is brought to the orifice, the more satisfactory is' thedeposition of selenium. If the operation is carried out underconventional atmospheric conditions, some of the selenium oxidizes andsome of the selenium condenses to ne red particles. The greater theplate distance, the greater is the ratio of oxidized and condensedselenium to selenium still in vapor form. Too large a proportion ofselenium oxide and selenium condensed to the red particle form causesthc selenium to peel and crack from the base plate during later heattreatment and may also lead to high resistance in the forward direction.

Other factors which influence the characteristics of the depositedselenium are: speed of the plate past the boiler orifice, rate ofselenium iiow and number of passes made by the plate past the orifice.All of the above factors must be correlated to give the most desirabletype of selenium deposition.

The temperature of the plate or plates upon which selenium is to bedeposited has not been found to be critical, but plates maintained at atemperature of between about 60 to 100 C. are satisfactory and yield asmooth black deposit of selenium, which is desirable in the manufactureof rectifier elements.

Figure 3 illustrates a modification in which a long or continuous bandof metal 31 is led over a series of selenium boilers B', which may beheated in any manner, as with electric resistance heating coils 39. Theboilers may all be identical in construction or may differvas to widthoi' orifice or other factors to superimposed layers of differentthickness or other characteristics. After coating, the strip may be cutinto rectifier elements of the desired dimensions.

Figure 4 shows a boiler modification wherein a gaseous atmosphere isintroduced by a conduit 38 into a boiler B2, the upper portion of whichis heated by a high frequency inductance coil 40. The heat travelsdownward to vaporize the selenium but the upper part of the boiler iskept hottest to prevent condensation. The gaseous atmosphere enteringthrough conduit 38 may comprise an inert gas such as argon, a relativelyinert gas such as nitrogen or carbon dioxide, or a reducing gas. Theboiler may be alternatively heated by gas, by radiant heat from anelectrical heater, by conducted heat from an electric heater, byelectromagnetic induction, or by other convenient means.

Figure 5 illustrates a two compartment boiler, wherein compartment 44 isadapted to receive so. lidified selenium or melted selenium at lowtemperature and compartment 46 is adapted to vaporize molten seleniumflowing from compartment 44. A valve 48 is disposed in the base of the'boiler to drain oiT the selenium or the accumulation of excessiveamounts of impurities.

In the embodiment shown in Figure 6, molten heated selenium may beintroduced into a boiler 50 from a vessel 5l, through a needle valve 52.Suiiicient heat is applied to 50 to cause the selenium to vaporize andflow through a slotted oriiice 54. The amount of selenium vaporized inthis type of boiler can be accurately controlled by the setting of theneedle valve 52 and the pressure behind the selenium in the vessel 5l.In

slide 60. When the selenium vapor issues at the desired rate from eachorifice the plate 56 is moved past the orifices in a manner similar tothat described in connection with Figure 1.

Figure 8 shows a method for applying a metal counter-electrode on top ofthe selenium deposit 62 on a metal plate 6I. A mask 63 having aperturesor windows 64 is placed against the selenium surface and a soft,low-melting-point metal 68 is sprayed onto the exposed areas of seleniumby the metal spray gun 65 supplied with com pressed air by air hose 66and containing molten metal heated by an electric heating elementsupplied by wires 10.

Figure 9 shows a plate after spraying, comprising backing metal plate 6|with selenium coating 62, and several sprayed metal electrodes 12 on theselenium surface. This plate may 'be cut into individual rectifierunits.

Figure 10 shows an improved plate wherein the backing plate 19 isnotched at intervals before the selenium. layer 18 is applied. Afterspraying of electrodes 'I2 the strip can be readily broken or cut at thenotches.

The sequence of steps or operations in the production of rectifierelements according to this invention is substantially as follows:

(1) The metal strips to be coated with selenium are grooved by means ofa kick press or automatic power press.

(2) They are then cleansed in any suitable manner, e. g. by immersion ina cleaning solution, a cleaning vapor, or wiping with a rag.

(3) Both sides of the metal strips are then sand-blasted, as forexample, with Carborundum of a particle size of 240 to 280 mesh, themetal strips being driven between rollers at a linear speed of about 10ft. per minute, one sand blast being directed against each side.

(4) The next step is the selenium deposition substantially as describedabove.

(5) The selenium plated strips are next heat treated at a temperature ofabout 210 C. for a period ranging from 15 minutes to one hour.

(6) A blocking layer is then applied to the plates, this consisting of athin film of selenium oxide. This may be effected, for example, byexposing the plates to selenium oxide vapors in a closed chamber for afew seconds, or by any other suitable method.

('7) The plates are next sprayed with a metal or alloy having a meltingpoint below the melting point of selenium.

(8) 'I'he final operation is the forming treatment, which consists insubjecting the plates to a suitable direct current, applied throughcontacts engaging the metal base plate and the sprayed metal layer in adirection in which the element tends to block current flow. Thisinverse: current is applied for a period of about 2 to hours, at acurrent density of, for example, about 300 milliamperes per sq. inch ofactive area.

While certain embodiments of the invention have been specificallyillustrated and described, it is understood that the invention may beotherwise embodied and practiced without departing from the principlesor scope of the invention as set forth in the appended claims.

What is claimed is:

1. The method of depositing a layer of selenium of substantially uniformthickness upon a metal surface which comprises the steps of vaporizingselenium at substantially atmospheric pressure, causing the resultingvapors to assume the shape of a thin ribbon, surrounding said streamwith an envelope of gas selected from the group consisting of reducinggases and inert gases, and establishing relative transverse movementbetween said ribbon of gas and said surface to effect the condensationand deposition of a gray, metallic layer of selenium upon said surface.f

2. In the art of depositing a selenium layer of substantially uniformthickness upon at least one surface of a metal base, the improvementwhich comprises the steps of vaporizing selenium at substantiallyatmospheric pressure, forming the resulting vapors into a4 thinribbon-like. stream in substantially unadulterated vaporized form,surropnding said stream with an envelope of gas selected from the groupconsisting of reducing gases and inert gases, moving said metal base soas to intersect said stream in a direction perpendicular to the widestdimension of said stream to form a thin, uniform and adherent coating ofselenium in the gray, metallic state upon said surface, and repeatingsaid movement back and forth through said stream to form a seleniumlayer of desired thickness.

3. Apparatus for depositing a relatively thin layer of selenium upon ametal base comprising, in combination, means for vaporizing and forminga thin ribbonlike stream of selenium vapor substantially at atmosphericpressure, means for supporting and bringing a base into contact withsaid stream of selenium vapor, and means for mixing a gas selected fromthe group consisting of vinert gases and reducing gases with said streamof selenium vapor.

4. Apparatus for depositing a relatively thin layer of selenium upon ametal base, said apparatus including a selenium vaporizer operablesubstantially at atmospheric pressure having a relatively narrowslot-like orifice disposed in the upper portion thereof, means forexternally heating said vaporizer to generate a stream of selenium vaporand to avoid condensation of said vapor in said orifice.

5. A method of depositing a layer of selenium upon a metal base whichcomprises vaporizing selenium at substantially atmospheric pressure,causing the resulting vapors to assume the shape of a stream,surrounding said stream with an envelope of gas selected from the groupconsisting of reducing gases and inert gases, and directing said streamagainst a metal base to form a uniform adherent coating of gray metallicselenium thereon.

6. Apparatus for depositing a layer of selenium upon a metal base whichcomprises a selenium vaporizing furnace having a narrow slotlikedischarge orifice, means forapplying heat to said furnace adjacent saiddischarge orifice to prevent condensation of selenium vapors in theregions near said orice, and a guide for supporting a metal base apreselected distance from said orifice.

7. Apparatus for depositing a layer of selenium upon a metal base whichcomprises a selenium vaporizing furnace having a narrow slotlikedischarge orifice, a guide for movably supporting a metal base apreselected distance from said orifice, said guide being accessible forinsertion of the base therein or removal of the base therefrom duringthe operation of said furnace,

and means for reciprocating said base to apply successive coatings ofselenium thereto.

8. Apparatus for depositing a layer of selenium upon arnetal base whichcomprises a selenium vaporizing furnace having a narrow slotlikedischarge orifice, and a pair of slotted guide members at oppositerespective sides of said orifice to slidably receive and support a metalbase a preselected distance from said orifice, whereby said base may bemoved along the slotted portions of said guides during operation of thefurnace.

9. Apparatus for depositing a layer of selenium upon a metal base whichcomprises a plurality of selenium vaporizing furnaces each having anarrow slot-like discharge orice, and a guide common to all of saidfurnaces for passing a metal base, in succession, a preselected distancefrom the respective discharge orifices whereby a coating of selenium isapplied to the base by the gases discharged from each furnace.

10. In the art of depositing a selenium layer of substantially uniformthickness upon at least one surface of a metal base, the steps whichcomprise vaporizing selenium at substantially atmospheric pressure,forming the resulting vapors into a thin rbbonlike stream insubstantially unadulterated vaporized form, directing said stream of gasagainst a metal base while eflecting relative transverse movementbetween said stream and said base to deposit a thin adherent coating ofselenium on one surface of said base, and maintaining a protectivegaseous envelope around said stream of selenium and said surface whilethe deposition is carried out to inhibit the formation of oxides ofselenium, said envelope being formed by a gas selected from the groupconsisting of inert gases and reducing gases.

11. In the art of depositing a selenium layer of substantially uniformthickness upon a surface of a metal base, the steps which comprisevaporizing selenium at substantially atmospheric pressure, forming theresulting vapors into a thin unadulterated stream of gas, moving a metalbase transversely through said stream to deposit an adherent vseleniumcoating on at least one surface thereof, and surrounding said stream andsaid surface W'lth a gas selected from the group consisting of inertgases and reducing gases to prevent formation of oxides of selenium.

C. SCUDDER SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS France Nov. 10, 1936

